The Zanzibar Land soldiers were the main forces of the country of Zanzibar Land, established in 1997. Like Outer Heaven, many soldiers were former mercenaries, who had participated in Zanzibar Land's War of Independence. By November 1999, the Zanzibar Land Forces had 30,000 soldiers at its command, which rapidly increased by to 40,000 in the next month (the time of Operation intrude F014). 24,000 of these soldiers were stationed as the security detail within their stronghold. Because of their strong patriotism for their country, they stop what they are doing and stand at attention when the Zanzibar Land national anthem starts playing, a character trait that Solid Snake exploited to get through various soldiers via a recording of the anthem. After Zanzibar Land's ambitions were thwarted by Solid Snake, the military was presumably left in shambles, after the country's collapse. It is unknown what happened to the remaining soldiers.

The Genome Soldiers, officially known as the Next-Generation Special Forces, were a genetically enhanced special forces unit, so called due to the gene therapy they had undergone. Members were injected with so-called "soldier genes," which heightened their senses and skills in combat. Because of this, and much to Solid Snake's dismay, most Genome Soldiers had little to no actual combat experience. It was theorized that the so-called Gulf War Syndrome was partly due to the side effects of Genome Soldier testing.

The Gurlukovich Mercenaries were a Russian mercenary unit founded by ex-GRU colonel Sergei Gurlukovich. It consisted of soldiers who had nowhere to go following the collapse of the Soviet Union, and continuously fought for their country's return to power.

The Gurlukovich Mercenaries were also trained as Arsenal Tengu Commandos in order to protect the inner sanctums of Arsenal Gear from infiltration and destruction. The Tengu soldiers had masks inside their helmets that can help them breathe in biochemical environments. They could also run faster and were stronger than the normal soldier, due to the artificial tissue they had in their armor.

The Ocelot Unit were an elite Spetsnaz GRU unit active during the 1960s. Armed with more powerful weapons than the ordinary GRU soldiers, they were very effective at fighting as a team. They were led by none other than Major Ocelot, who would later become Revolver/Liquid Ocelot.

The San Hieronymo Unit was a Red Army special forces unit that was deployed to the San Hieronymo Peninsula in order to work on a Soviet Missile Base, but was ultimately abandoned by the Soviet Union, due to both Détente and the SALT talks that both Leonid Brezhnev and Richard Nixon were promoting. In 1970, they ended up joining Gene's rebellion so that they could build a nation for soldiers. At least some members of the unit joined Big Boss to put a stop to Gene.

The FOX Unit was deployed to San Hieronymo Peninsula in 1970. Following the FOX rebellion, many of them patrolled around the peninsula, and guarded special areas. Two FOX soldiers were killed by Null when he escaped from his culture tank and went after Gene. Many soldiers also went with Gene to stop Snake from destroying Metal Gear, but were killed by Colonel Skowronski in Metal Gear RAXA. At least a few members of the unit joined Big Boss to put a stop to Gene.

By 2014, private military companies were beginning to overtake small armies in terms of firepower and used by larger nations to avoid any political controversy that involves the armed forces of their own. It is also stated that several state armies were being reduced in number due to the effectiveness of the PMC forces. They consisted of "guns-for-hire", which were employed for combat and fought without the thought of countries or ideologies or ethnicity. This was well in tune with the war economy as money was the only thing PMCs fought for. PMCs did not just contain human soldiers, but also utilised technology such as the Gekko, provided by AT Corp.

In 2014, the five largest PMCs on the planet (together rivaling the U.S. Army in numbers and firepower) were owned by a single mother company, Outer Heaven, led by Liquid Ocelot. Logistically, technologically, and tactically advantaged, they were deployed all over the globe with the use of high-end technology on the battlefield, and could come in and out of any conflict with minimal casualties. PMCs were also equipped with gear and weaponry used by the U.S. military itself such as SCAR rifle, M4A1s (at least in earlier renders), Humvees, Strykers and numerous more.

The Peace Sentinel were a rogue unit of the CIA created to organize Project Peace Walker. They were led by CIA Station Chief in Central America, Hot Coldman. In 1973, they were deployed into Costa Rica in order to set up the nuclear weapon "Peace Walker" and launch it as their last nuke for deterrence, and create the perfect deterrent.

The Sentinel came in various varieties: Some as armored soldiers (Kazuhira Miller referred to them as reinforced soldiers), some as CIA Mercs (distinguished by their blue pants, golfer hat, and orange jacket, along with a hunting rifle), as well as soldiers wearing green uniforms and balaclavas. If an alert was triggered, there was also a subset of Peace Sentinel soldiers (presumably security or SWAT-like members) that appeared, wearing blue uniforms, a balaclava, headphones, as well as goggles where their eyes should be. It is unknown if there were any distinctions based on rank in most of these units, although it is known that the Reinforced Soldiers had one different variety for the commander of that particular unit. The commander of one of the Reinforced Soldier Units often had a red device adorning his helmet, and more often than not commandeered a vehicle. There were also a variety of soldiers that had brown moustaches, white T-shirts, green pants, and headsets, and sometimes were wearing shades and were clean shaven. These men were actually guards for the Underground Facility where Basilisk was stored.

There were also a variety of soldiers who wore uniforms similar to Strangelove, who were also of Soviet origin, along with a tan-helmeted, blue outfitted variety during Alerts. These were actually members of the Soviet military that Vladimir Aleksandrovich Zadornov supplied the Peace Sentinel in terms of their alliance, to which they later betrayed.

Desperado and World Marshal were PMCs based in Delaware (formally) and Denver, Colorado respectively. Desperado was encountered throughout the entire game while World Marshal was encountered by Chapter R-03.

Soldiers, as a general standard, carry radios for constant contact with their superiors. In later games, if a player is able to disarm a guard before he calls for help, shoot the radio or jam it with a chaff grenade, then the soldier would not be able to call for reinforcements. However, even if the player manages this, the soldier can still respond by escaping to a safer area and then call for reinforcements. Soldiers manning high points are also known to carry binoculars, giving them a better field of vision than others.

Soldiers, in addition to the regular patrols, are also seen in attack squads, which may have slightly different weapons or equipment. Usually seen exclusively in Alert, Evasion, and Caution mode, these soldiers will swarm the player and either attack him in droves or (in Evasion mode) perform a "clearing", during which they actively fill a room they believe the player is hiding in and aim to expose his hiding place. This will even include checking inside lockers and attacking any cardboard boxes.

Typically, if enough of the attack squads are incapacitated (knocked out, tranquilized, or killed outright) while the game will still go to Alert Mode on detection, no reinforcements will arrive (due to all such reinforcements being dead or otherwise incapacitated at the time).

Although the soldiers occasionally are alerted to the presence of the player with stuff like grenades, glitches sometimes occur where they dismiss the presence of the player despite clear evidence of the player's presence. For example, in Peace Walker, sometimes even if the grenade detonates seven feet away from an enemy soldier after making enough noise from landing impact to have the soldier hear it, he'll dismiss it as being "some kind of animal."

After Metal Gear Solid, where the Genome Soldiers iconically wore balaclavas, enemy soldiers in subsequent games are typically shown wearing balaclavas (and/or some other form of covering headgear) as part of their uniform as well. Metal Gear Solid V: Ground Zeroes was the first game to feature absolutely no balaclava wearing enemies.